As the number of wireless and wired electronic devices increases in residential and office environments, so does the likelihood of interference amongst such devices. Various wireless connections can cause such interference such as cell-phone to base station connections, WiFi-connections, Bluetooth connections, inductive coupled connections RFID based connections, and the like. In a number of these systems (such as cell-phone, WiFi and Bluetooth) the connection occurs through narrow pulses in time domain. Thus, the power dissipated to initiate and establish the connection is minimized. The inductive couple connections and RFID recognition systems of the type that include an interrogator and a transponder (RFID interrogation connection) are popular; and, used for toll collection and inventory control respectively.
With inductively coupled transmitter-responder arrangements, an interrogator generates an AC power field and a receiving responder tag may be positioned at a preselected position. The interrogator generated AC power is received by the responder tag through inductive coupling; and, the responder tag is activated. A uniquely coded signal particular to that tag can then be generated. In this type of transmitter-responder arrangement, the magnetic field is utilized for responsiveness. This system relies on near-field interaction and thus the radiation nature of the radio signal is ignored. With such arrangements, magnetic fields do not diminish quickly enough with distance and are not suitable in smaller physical spaces such as offices and residences.
RFID, instead of relying only on the magnetic fields, relies on electromagnetic energy for activation. Like inductive coupled systems, typical RFID recognition systems contain an interrogator (the first unit) and at least one tag (also referred as a transponder or the second unit). The tag or transponder rectifies the RF electromagnetic field in its vicinity and depending on the RF power strength may change its state. The RF field is generated by the interrogator which is thus able to control the tag. The amount of energy decreases as the tag goes away from interrogator. The received RF field at the transponder is critical in determining the behavior of the transponder.
The radiation pattern near the interrogator is characterized as near-field, while the radiation pattern away for the interrogator a far-field. When the transponder is near-field, the received energy strength can change substantially with slight displacement. In the far-field the received power by the transponder is more deterministic given by the following equation:
                              P          Rx                =                                            P              Tx                        ⁢                          G              Tx                        ⁢                          G              Rx                        ⁢                          λ              2                                                          (                              4                ⁢                π                ⁢                                                                  ⁢                D                            )                        2                                              (        1        )            
In equation (1), PRx is the received power at the transponder, PTx is the transmitted power of the interrogator, GTx is interrogator's transmitter antenna gain, GRx is transponder's receiver antenna gain, D is the distance between the receiver and transmitter, and λ is the wavelength. In this equation PTx, GTx and GRx are fixed quantities determined by interrogator output power, transmitter antenna gain, and the receiver antenna gain respectively. Equation 1 is valid when the receiver antenna is in far-field region of the transmitter antenna. At 3 GHz, the wavelength of wireless signal is about 10 cm in air. So as the distance between the interrogator and transponder increases from 10 cm to 20 cm the signal strength at the receiver drops by ¼ (or 6 db) and from 20 cm to 40 cm it drops by another ¼ and so on. The signal strength decreases by 12 dB when distance is increased four folds. Equation (1) is invalid when there is significant scattering. The above equation is valid for all frequencies ranging from low microwave frequency to high millimeter wave frequencies.
In some toll booth systems, a threshold detector is in communication with an antenna to measure the power level of an RF interrogation. When the power level is greater than a certain threshold, the system initiates further testing using modulated signals to verify the present modulation state. When both of these states meet predefined conditions, a corresponding transponder can be enabled. Such arrangements are not appropriate for office or residential applications.
In a typical office environment there are many wireless signals. To name a few, cordless phones at 900 MHz, 1800 MHz and 5000 MHz, Cell phone between 0.9 GHz-2 GHz, Bluetooth in 1-2 GHz, Gaming devices, Computer generated noise and many others. These signals can increase background noise and cause interference. A pre-selection filter is therefore needed to remove the spurious signals. However, such filter can be prohibitively expensive for consumer electronic use. Alternatively, the transmit power PTx and thus the threshold be increased so that all of the interference is made comparatively small. However, the increase of PTx is not possible because of Federal Communication Commission (FCC) and other regulation. In addition, in a home or office environment there maybe plethora of transponder type devices, all of the devices would see the interrogator power. The transponder (or the second unit) would all be switched on simultaneously in response to the interrogators signal power. This, therefore, results in significant power waste.
Conventional techniques for selectively initiating communications are complicated when there are a large number of transceiver units within a relatively small area (such as residences and offices).